Electric traction systems and vehicles comprising such systems

ABSTRACT

An Electric traction system having a separate excitation d.c. motor, an armature current control device, an excitation current control device, and between the motor armature and the d.c. supply source, several armature current switches (4-1...4-n) in parallel each made up of one or more transistors connected in parallel (30, 31 32). In order to avoid possible non-synchronous operation of the switches, each is connected with one terminal to an induction coil (5-1...5-n) and in series with the motor armature and with its other terminal to a diode (8-1...8-n) shunting said induction coil and the motor armature.

United States Patent 1 1 Lecoeuche et a1.

[451 Mar. 18, 1975 1 ELECTRIC TRACTION SYSTEMS AND VEHICLES COMPRISING SUCH SYSTEMS [75] Inventors: Jacques Lecoeuche; Gerard Lacroux, both of Malakoff, France [301 Foreign Application Priority Data 7/1973 Steinmann .1 318/341 10/1973 Morton et a1 318/139 Primary Examiner-Robert K. Schaefer Assistant E.\'aminerW. E. Duncanson, Jr. Attorney, Agent, or Firm-Roland Plottel, Esq.

[57] ABSTRACT An Electric traction system having a separate excitation d.c. motor, an armature current control device, an excitation current control device, and between the motor armature and the dc. supply source, several armature current switches (4-l...4-n) in parallel each made up of one or more transistors connected in parallel (30, 31 32). In order to avoid possible nonsynchronous operation of the switches, each is connected with one terminal to an induction coil (5-1 ...5- n) and in series with the motor armature and with its other terminal to a diode (8-l...8-n) shunting said induction coil and the motor armature.

10 Claims, 7 Drawing Figures 1 ELECTRIC TRACTION SYSTEMS AND VEHICLES COMPRISING SUCH SYSTEMS The present invention relates to improvements in electric traction systems and vehicles comprising such systems.

Known electric traction systems usually resort to series type electric motors in which the inductive flow which is proportional to the armature current, makes it possible advantageously to obtain a considerable motor torque upon the appearance of a resistant torque. However, the inductive flow depends in the functioning of these motors, on a single parameter, the armature current, and the torque-speed characteristic of these motors, determined during the design of these latter remains invariable. The operation of such electric traction systems proves to be relatively rigid.

The present invention aims at avoiding this drawback and permitting the achievement of an electric traction system comprising a separate excitation d.c. motor and control devices for the armature current and excitation current which, by linking the armature current and excitation current of the motor to several parameters, gives a great facility in modification of the functioning characteristic of the motor, consequently, a great flexibility in the behavior of the system. The invention further has for its object an application of such an electric traction system to a vehicle.

An electric traction system according to the invention comprising a separate excitation d.c. motor, a device for controlling the armature current and a device for controlling the excitation current, is characterized in that it comprises at least, between the motor and the dc. source, a group of several transistors in parallel acting as a switch of the armature current. A vehicle according to the invention is characterized in that it comprises such an electric traction system.

For a better understanding of the invention, there is given by way of indication an embodiment, shown by the accompanying drawings, in which:

FIG. 1 represents a partial, diagrammatic view of an electric traction system according to the invention showing the armature current control device;

FIG. 2 represents a partial, diagrammatic view of the electric traction system of FIG. 1, showing the excitation current control device;

FIG. 3 represents a partial, diagrammatic view of the electric traction system of FIG. 1, showing an armature current switch;

FIG. 4 represents a partial, diagrammatic view of the electric traction system of FIG. 1, showing a pulse generator 9 of the armature current control device;

FIG. 5 represents a partial, diagrammatic view of the electric traction system of FIG. 1, showing a threshold pulse generator 11 of the armature current control device;

FIG. 6 represents two voltage diagrams, the one at the inputs and the other at the output of the threshold system 72 of the excitation current control device; and

FIG. 7 represents a partial, diagrammatic view of the energy recovery system during braking of the electric traction system of FIG. 1.

An electric traction system according to the invention shown in FIGS. 1 and 2 comprises a separate excitation d.c. motor whose armature is indicated by 1 (FIG. 1) and field winding is designated by 2 (FIG. 2), an armature current control device (FIG. 1) and an excitation current control device (FIG. 2).

The armature 1 of the motor is fed by a dc. source 3, storage batteries, for example. According to an important characteristic of the invention, armature l is fed current through one or more switches in parallel 4- l...4-n and each of these switches is in series ahead with an induction coil 5 and fuse 6 and below with a corresponding resistance 7. Each of the units of armature 1 induction coil 5 fuse 6 is shunted by a diode 8. Consequently, for n switches, there are n induction coils 5-1...5-n, n fuses 6-1...6n, n resistances 7-1...7n and n diodes 8-l...8n.

When a switch 4-1, for example, is closed, a current circulates from source 3, through armature 1, induction coil 5-1, fuse 6-1, switch 4-1 and resistance 7-1 to return to source 3. The energy thus taken from source 3 in this circuit comprises two parts, a part used in the motor and a part stored in induction coil 5-1. When switch 4-1 is opened, diode 8-1 becomes conductive and the energy stored in the induction coil 5-1 is sent into the motor. The motor receives the energy for a time t of closing of switch 4-1, and fora time t of restoration of the energy by induction coil 5-1 when switch 4-1 is opened. Thus, during the time t t, of functioning of the motor, this latter takes energy from source 3 only during time I.

Since armature current switches 4-1...4-n are in parallel and each comprises its own induction coil 5 and its own diode 8, their functioning does not have to be perfectly synchronous; this would not be the case with a single induction coil common to all switches 4-1...4-n.

The advantage made bythe use of several switches 4-l...4-n in parallel above is important because if one of these switches no longer functions, the other switches continue to assure feeding of motor 1 and the traction system is not stopped.

The armature of a separate excitation motor usually presents a slight resistance of the order of some tens of milliohms and a negligible induction coil. If the motor is blocked and fed directly by current source 3, the current going through the armature, known as a shortcircuit current, is enormous, of the other of several thousand amperes. Without-the presence of induction coils 5, switches 4-1...4-n would have to have a large gauge to support this current peak. With induction coils 5- 1...5-n described above, switches 4-1...4-n can have a slighter gauge, calculated in relation to the normal operating current of the motor and not with its shortcircuit current.

Opening and closing of switches 4-1...4-n are assured by three pulse generators 9, 10, 11 and an electronic flip-flop 12.

Electronic flip-flop 12 of known type, having two inputs l3 and 14 and two outputs l5 and 16 comprises two stable electric states. In its first stable state, its output 15 is at zero potential and its output 16 at the potential of supply source 3. In its second stable state, its output 15 is at the potential of the supply source 3 and its output 16 at zero potential. A pulse applied to its input 13 puts flip-flop 12 in its first state while a pulse applied to its input 14 puts flip-flop 12 in its second state.

Flip-Flop 12, which is controlled by the pulses delivered by pulse generators 9, 10, 11 in its turn controls, through its output 16, the opening and closing of switches 4-l...4-n. These switches close when potential 16 of flip-flop 12 is equal to zero and open when 16 has the potential of source 3. Pulse generator 10 is a known type generator which delivers, at its output 17, fixed frequency pulses. A switch 18 makes it possible to start functioning. The pulses of generator are applied to input 14 of flip-flop 12 to cause the closing of switches 4-1...4-n. Pulse generator 10 will hereafter be called the closing generator.

Generator 9 according to the invention is a pulse generator whose functioning can be blocked automatically upon-the appearance of certain predetermined states of flip-flop 12 and/or of generator10, and which delivers, at its output 21, pulses with a frequency that can be regulated by the conductor of the traction system from a high frequency, greater than the fixed frequency of the pulses of generator 10, to a low frequency less than said fixed frequency. Regulation of this frequency can be done, for example, through a device comprising a potentiometer 19 whose sliding contact is connected to a pedal 20. When is at rest, the frequency of the pulses of 9 is greater than that of the pulses of 10. The frequency of the pulses of 9 diminishes as pedal 20 is depressed. The pulses of 9 delivered, at its output 21, are applied to input 13 of flip-flop 12 to cause opening of switches 4-1...4-N. Pulse generator 9 will hereafter be called the opening generator.

Opening generator 9 comprises, actually, besides its output 21 a terminal 22 connected to output 15 of flipflop 12 and a terminal 23 connected to output 17 of generator 10.

Opening generator 9 is blocked and does not deliver pulses when its terminal 22 is at zero potential. It is unblocked and delivers pulses when its terminal 22 is at the potential of supply source 3. Opening generator 9 is also blocked when its terminal 23 receives a pulse from closing generator 10 and, in this case, does not deliver any pulse at its output 21.

When a pulse delivered by closing generator 10 reaches input 14 of flip-flop 12, it puts this flip-flop 12 in its second'state, wherebyits output 15 is at the potential of supply source 3 and its output 16 is at zero potential. Switches 4-1...4-n close, output 16 of flipflop 12 being at zero potential and opening generator 9, having its terminal 22 at the potential of output l5 of flip-flop '12 which is, at that moment, at the potential 0f the supply source, is-unblocked and delivers, a time I later, at its output 21, a pulse which, arriving at input 13 of flip-flop 12, makes this latter go into its first state. Output 15 of flip-flop 12 thus coming back to zero potential, opening generator 9 is again blocked and therefore delivers in the meantime, a single pulse. Output 16 of flip-flop 12 refinding the potential of supply source 3, switches 4-l...4-n open. The cycle begins again with the arrival of a pulse from closing generator 10 at input 14 of flip-flop l2.

Switches 4-1...4-n thus closing at each pulse of closing generator 10, i.e., all T seconds, T being fixed by the fixed frequency of the pulses, of this generator 10. These switches open, after 2 seconds, at the arrival of a pulse from opening generator 9 at input 13 of flip-flop 12. Switches 4'-l...4-n therefore are closed during time t and open during times T-t.

Time I is slightwhen pedal 20 for regulating the frequency of the pulses of opening generator 9 is at rest and this time 1 increases as this pedal 20 is operated, i.e., depressed by the conductor of the traction system.

When time t is less than T, the pulse from closing generator 10, arriving at terminal 23 of opening generator 9 does not produce any effect because generator 9, having delivered a pulse, meantime, is already blocked, since its terminal 22 is at zero potential. But when t becomes greater than T, the pulse from closing generator 10 reaches terminal 23 of opening generator 9 and blocks this latter, because generator 9 is unblocked meantime. Opening generator 9 does not deliver any pulse. Flip-flop 12 remains in its second state and switches 4-1...4-n remain closed. A permanent closing of switches 4-l...4-n is thus obtained.

Pulse generator 11 assures protection against any overload of switches 4-1'...4-n. Hereafter, it will be called the protective generator. Protective generator 11 is, according to the invention, a threshold and blockable pulse generator. Protective generator 11 delivers pulses at its output 24 only if its input 25 is at a potential greater than a predetermined potential known-as the threshold potential. It is blocked when its terminal 26 is at zero potential. According to the invention, protective generator 11 has a threshold potential that can be regulated by means of a potentiometer 27, for example (FIG. 1). Its output 24 and its terminal 26 are respectively connected to input 13 and output 15 of flip-flop 12 so that 11 is blocked when switches 4- 1...4-n are open, 15 of flip-flop 12 being at this moment at zero potential.

Input 25 of generator 11 is connected to common point 28 of the cathodes of n diodes 29-l...29-n whose anodes are respectively connected to the junction points of resistances 7-1...7-n and switches 4-l...4-n. When switches 4-l...4-n are closed, at the terminals of resistances 7-1...7-n there appear voltages proportional to the currents going through these switches, respectively. Diodes 29-l...29-n make it possible to send the greatest of these voltages to input 25 of protective generator 11. As soon as this voltage is greater than the threshold voltage of protective generator 11, a pulse is delivered by this generator 11. This pulse, sent into input 13 of flip-flop 12, changes the electric state of this flip-flop 12 by giving back to output 16 of this latter the potential of source 3, which has the effect of opening switches 4-1...4-n. The protection against overload of these switches is thus assured. Regulation of the critical strength of the current in switches 4- 1...4-n is-thus made. by a simple adjustment of the threshold voltage in protective generator l1.

According to a preferential embodiment of the invention, the armature current switches 4-1...4-n are eachmade up' of one or more transistors in parallel. In the example shown in F163, a switch 4-1 is made up of three transistors in parallel 30, 31, 32. This switch is in series, on the one hand, on the side of the collectors of these transistors, with an induction coil 5-1 and an armature 1 of the motor, these latter being shunted by a diode 8-1 and, on the other hand, on the side of the emitters of these transistors with resistances 33, 34, 35. Diodes 36, 37, 38 associated with resistances 33, 34, 35 assure the greatest selection of voltages proportional to the currents going through switch 4-1, i.e., transistors 30, 31, 32 to send it to input 25 of protective generator 1 1. Transistors 30, 31, 32 in parallel are controlled by a transistor 39, itself controlled by output 16 of flipflop 12 through two diodes 40 and 41 shunted by a capacitance 42. The base potential of 39 being fixed by resistance 43.

Opening generator 9 is preferably madeup of a pulse generator represented in FIG. 4. This opening generator comprises a unijunction transistor 44 whose base 45 is connected by means of a resistance 46 to the negative pole of current source 3 and by diode 47 to output 21 of said generator, and whose base 48 in series with a resistance 49 is connected to the positive pole of source 3. Emitter 50 of unijunction transistor 44 is con- I connected by a diode 54 to terminal 23 of said generator 9. Potentiometer 19 is operated by a pedal 20 already mentioned in one of the above paragraphs. This pedal 20 is called hereafter the acceleration pedal of the traction system.

When terminal 22 of opening generator 9 is at the potential of the positive pole of source 3, capacitance 52 is charged through potentiometer l9 and the more rapidly as the value of 19 is slight, i.e., acceleration pedal 20 is depressed less. When the voltage at the terminals of 52 reach a certain value which depends on the characteristics of unijunction transistor 44, capacitance 52 rapidly discharges through emitter 50 base 45 region of unijunction 44 and resistance 46. Thus, a positive pulse is produced at the terminals of 46 and this pulse is transmitted through diode 47 to the output of opening generator 9.

When terminal 22 of generator 9 is at zero potential, in other words, at the potential of the negative terminal of source 3, diode 51 short circuits capacitance 52 which cannot charge and there is no production of pulses at output 21 of opening generator 9.

In case, during charging of capacitance 52, a pulse is applied to terminal 23 of opening generator 9, transistor 53 becomes conductive and discharges capacitance 52. In this case, there is no longer any production of pulses at output 21 of opening generator 9.

Protective generator 11 is preferably made up of a pulse generator represented in FIG. 5. This protective generator 11 comprises, as in opening generator 9, a unijunction transistor 55 with its bases 56, 57 connected respectively to the negative and positive poles of source 3 through resistances 58 and 59, with its emitter connected to the negative pole of the source through capacitance 61, a terminal 24 connected to base 56 of unijunction 55 by means of a diode 62 and a terminal 26 connected to the emitter of unijunction 55 by a diode 63, transistor 53 shunting capacitance 52 being eliminated and potentiometer 19 being replaced by a resistance 64. Further, a potentiometer 27, connected to the terminals of source 3, is connected by its slide contact to the emitter of unijunction 55 by a diode 65 and a transistor 66 connects, by means of its emitter-collector resistance a resistance 67 in series, base 57 of unijunction 55 to the negative pole of source 3, the base of transistor 66 being connected to input of generator 11 through a resistance 68. Input.25 of generator 11 is connected to the negative pole of source 3 by a resistance 69.

Potentiometer 27 fixes the threshold voltage of protective generator 11. The potential of input 25 being zero, capacitance 61 is charged through 64. When the voltage at the terminals of capacitance 61 reaches the potential of the slide contact of potentiometer 27 diode 56 becomes conductive and limits the charge of the capacitance 61 to this value.

If the voltage applied to input 25 increases, transistor 66 becomes more and more conductive and makes the potential of base 57 of unijunction 55 drop. When this potential reaches a certain value, determined by the potential of the slide contact of potentiometer 27, unijunction transistor 55 suddenly becomes conductive and capacitance 61 discharges into resistance 58 through emitter 60 base 56 region of this unijunction transistor. There is then a production of a pulse which is sent to the output 24 of protection generator 11.

The excitation current control device of the motor of the electric traction system of the invention is shown in FIG. 2. This control device makes it possible to modify, at will, the value of the excitation current, subject the excitation current to the armature current to increase automatically the motor torque upon the appearance of a resistance torque, and to link the excitation current to the speed of rotation of the armature to make it possible to obtain, at will, beyond a certain predetermined rotation speed of the motor, an acceleration of the operation of this motor.

In an example of preferred embodiment, shown in FIG. 2, the excitation winding 2 of the motor is connected to source 3 through a switch made up of a transistor 70. The collector of the transistor 70 is connected to a terminal of excitation winding 2, the other terminal of this excitation winding being connected to the positive pole of source 3, and the emitter of transistor 70 is connected to the negative pole of source 3. A diode 71, known as the recovery diode, shunts excitation winding 2.

Closing and opening of the switch made up of transistor 70 are, according to an important characteristic of the invention, assured by a unit comprising a threshold system 72 whose output 73 controls the base of transistor 70 and whose input 74 is connected to a system assuring a link with the armature current and a control of cyclic opening and closing of the excitation current switch and input 75 is connected, on the one hand, to a device for regulating the threshold voltage and, on the other hand, to a system assuring a link with the motor speed.

The system assuring a link with the armature current and the control of the cyclic opening and closing of the excitation current switch comprises a saw-toothed pulse generator 76, an amplifier 77 and an adder 78.

Generator 76 delivers a saw-toothed control voltage. In other words, the voltage delivered at output 79 of this generator is linearly increasing, starting from zero, then rapidly comes back to zero at the end of time T and the cycle begins again. The saw-toothed voltage of generator 76 is sent into input 80 of adder 78. Amplifier 77 comprises an input 81 which receives a voltage proportional to the armature current of motor 1 taken from the common point 28 of the cathodes of diodes 29-1...29-n (FIG. 1). The output voltage of amplifier 77 which charges a capacitance 82, is applied to input 83 of adder 78.

There is gathered at output 84 of adder 78, whose inputs 80 and 83 are respectively connected to the outputs of generator 76 ahd amplifier 77, an arimethical sum S (FIG. 6) of a saw-toothed voltage and a voltage proportional to the armature current of the motor, a sum of voltages sent to input 74 of the threshold system 72.

Input 75 of threshold system 72 receives, by means of a diode 85, a voltage that can be regulated by a potentiometer 86. This voltage is called the threshold voltage.

As long as the voltage applied to input 74 of-the threshold system 72 remains less than the threshold voltage applied to input 75 of this threshold system 72, output 73 of said threshold system has a zero potential and the switch made up of transistor 70 is open. For

any voltage value at 74 greater than the threshold volt- I age at 75, the potential at output 73 of the threshold system 72 becomes that of the positive pole of the source and the switch is closed.

The switch made up of transistor 70 is thus operated cyclically. A cycle is made up ofa time t of closing and a time T-t of opening, constant time T being the period of the saw-toothed voltage of generator 76. The longer time t, the greater will be the excitation current.

If the armature current increases voltage I, i.e., the voltage at 83 of adder 78 increases, the saw-toothed voltage undergoes a vertical translation toward the positive potentials and time tincreases (FIG. 6) for a given value X of the threshold voltage of 72. The switch remains closed longer during a period. The excitation current increases. The link between the armature current and the excitation currentis thus achieved. Permanent closing of the switch is produced when voltage I at input 83 of adder 78 exceeds the threshold voltage For a given voltage I at 83 of adder 78, i.e., for a determined armature current, time t varies with the value of threshold voltage X. It increases with a reduction of this threshold voltage and decreases with its increase. By regulating threshold potentiometer 86, Le, the threshold voltage, the value of the excitation current is regulated. Thus, a family of characteristic curves of the motor is obtained.

Another important characteristic of the invention consists in linking the excitation current to the rotation speed of the motor by modification of threshold voltage X. A potentiometer 87, whose slide contact is controlled by an acceleration pedal 20, the same that operates potentiometer 19 of opening generator 9 described above, and connected to input 75 of threshold system 72 by means of a diode 88, is mounted, fed by source 3 through another threshold system 89 which is controlled at its input .90 by a voltage proportional to the speed of rotation of the motor. This voltage comes from a tachymetric element 91 mounted on the shaft of armature 1 of the motor.

As long as the voltage applied to input of 89 is less than a predetermined value, the value corresponding to a given speed of the motor, potentiometer 87 is not fed, regardless of the position of acceleration pedal 20 and the voltage on slide contact of 87 is zero. Diode 88 is blocked. Threshold system 72 is controlled only by the threshold voltage coming from potentiometer 86. P- tentiometer 87 therefore has no action on the functioning of the device.

When the voltage at input 90 of threshold system 89 becomes greater than the above mentioned predetermined value, i.e., when the motor exceeds a certain given speed, potentiometer 87 is fed by thesource through threshold system 89. As acceleration pedal 20 is depressed, the voltage delivered by the slide contact 87 increases. But as long as its value remains less than the voltage coming from potentiometer 86, diode 88 remains blocked and the rotation condition of the motor is not changed.

If the acceleration pedal 20 continues to be de-- pressed, the voltage of the slide contact of potentiometer 87 becomes greater than that of potentiometer 86 and diode is blocked and diode 88 becomes conductive. The voltage of 87 replaces that of 86 to control threshold system 72.

This control voltage is the greater the more acceleration pedal 20 is depressed, and time t of closing of the switch, made up of transistor 70, will be slighter. The excitation of the motor diminishes. The motor is accelerated and reaches a high speed condition.

It is necessary that the acceleration pedal 20 have been sufficiently depressed and that the motor have reached a certain speed to accelerate the motor and to obtain a high speed condition. A sudden depression of acceleration pedal 20 at starting does not permit an acceleration of high speed condition because the motor at this moment is turning too slowly. Even with a high speed condition, the excitation current remains linked to the armature current, which permits an increase of the motor torque upon the appearance of a resistant torque. 1

In an application of the invention, the traction system described above is mounted on a vehicle. The conductor of this vehicle can act on pedal 20 to modify the speed of the motor of the traction system, i.e., the speed of the vehicle. At the same time, he can regulate potentiometers 27, 86 to obtain other operating conditions of the vehicle. The driving flexibility is conse quently great.

' The electric traction system according to the invention described in' the above paragraphs can further comprise an energy recovery system (FIG. 7) during the braking period; The energy is then transferred from motor 1, 2 which functions as a generator to do. source 3.

In a preferred embodiment (FIG. 7), this energy recovery system comprises, on the one hand, a diode 92, whose anode is connected to the negative pole of current source 3 and the cathode isconnected to the common point of induction coils S-l...5-n and the terminal of armature l of the motor, and, on the other hand, a subassembly made up of a voltage comparator 93, a potentiometer 94 whose slide contact is linked to abrake pedal 95 schematically represented by broken lines, an amplifier 96 and a connecting diode 97. Connecting diode 97 comprises a cathode connected to the cathode of diode 92 and the anode connected to an input of voltage comparator 93, the other input of the latter is connected to potentiometer 94. Voltage comparator 93, which is a known type, compares the voltage Vd at the terminals of diode 92 and that Vp coming from potentiometer 94. The resulting voltage of the difference of voltages Vp Vd, picked up at the output of the comparator 93, is amplified by amplifier 96 and applied to input 83 of adder 78 of the control device of the excitation current of the motor of the electric traction system, shown in FIG. 2.

In the electric traction system of the invention, when the acceleration pedal 20 is let up, switches 4-l...4-n are open.

At the time of braking, potentiometer94 is operated by brake pedal 95 so that voltage Vp coming from potentiometer 94 increases as this brake pedal 95 is depressed. The motor of the electric traction system of the invention functions then as a generator and excitation of the latter is a function of the output voltage of the terminals of diode 92 is fairly equal to the voltage Vp coming from potentiometer 94. Voltage Vd at the terminals of diode 92 is connected, according to the characteristic of diode 92, to the recovery current going through this diode 92. To each given voltage Vp coming from potentiometer 94, therefore to each position of depression of brake pedal 95, corresponds a determined recovery current. The more the brake pedal is depressed, the greater this recovery current.

Feeding of the voltage comparator 93 and amplifier 96 is preferably made through a switch 98 normally open whose closing is achieved when brake pedal 95 is operated.

What is claimed is:

1. Electric traction system comprising a separate excitation d.c. motor, an armature current control device, an excitation current control device, and between the motor armature and the d.c. supply source, several armature current switches (4-l...4-n) in parallel each made up of one or more transistors connected in parallel (30, 31, 32) characterized in that in order to avoid possible non-synchronization of the switches each switch is connected at one terminal to an induction coil (5-1...5-n) and in series with the armature of said motor, and at the other terminal with a diode (8-l...8-r'z) shunting said induction coil and armature of the motor.

2. Electric traction systemaccording to claim 1 wherein each of the armature current switches in series to its respective resistor (7-1...7-4) characterized in that in order to provide a signal to open the switches when a voltage proportional to the armature current passing through the switches is too large and exceeds a predetermined value, the system includes diodes (21- l...2 l -n) having like terminals connected to a common point (28) where the voltage proportional to the armature current may be detected, and whose other terminals are branched each being connected to one of the armature current switches between the switch and its series resistance (7-l...or 7-n).

3. Electric traction system according to claim 2 characterized in that the excitation current control device comprises, on the one hand, at least a transistor (70) acting as the excitation current switch and a threshold system (72) which, connected its output (73) to the base of the transistor (70) controls the conduction of said transistor, in other words, the opening and closing of this excitation current switch, and, on the other hand, a unit comprising at least a saw-toothedpulse generator (76), an amplifier (77) of the voltage proportional to the armature current, taken from the common point 28) of the cathodes of the diodes (36, 37, 38) mounted below the armature current switches and an adder (78) whose two inputs (80 and 83) are respectively connected to the output (79) of said sawtoothed generator and to the junction point between the charge capacitance (82) and the output of said amplifier (77) and whose output (84) is connected to the input (74) of this threshold system (72).

4. Electric traction system according to claim 3, characterized in that it comprises in the control of the threshold system (72), on the one hand, a potentiometer (86) giving the threshold voltage and whose slide 10 contact is connected through a diode to the input (75) ofa threshold system (72) and, on the other hand, a unit comprising a potentiometer (87) whose slide contact is controlled by an acceleration pedal (20) and connected through a diode (88) to the input (75) of the threshold system (72), a threshold system (89) feeding by its output (92) said potentiometer (87) as soon as a tachymetric element (91), connected to its input (10), displays a predetermined rotation condition of the armature (l) of the motor.

5. Electric traction system according to claim 3 provided with an energy recovery system during braking, characterized in that this energy recovery system comprises, on the one hand, a recovery diode (92) whose anode is connected to the negative pole of the current source (3) and the cathode is connected to the common point of the induction coils (5-l...5-n) and the terminal of the armature (1) of the motor and, on the other hand, a subassembly made up of a voltage comparator (93), a potentiometer (94) whose slide contact is connected to a brake pedal (95) and an input of this comparator (93), a connecting diode (97) whose anode is connected to another input of the comparator (93) and the cathode to the cathode of the recovery diode (92) and an amplifier (96) whose input is connected to the output of the comparator (93) and the output is connected to the input (83) of the adder (78) of the excitation current control device of the motor of the electric traction system.

6. Electric traction system according to claim 1 wherein (each of) the armature current switches has transistors (30, 31, 32) connected in parallel and each of said transistors is connected in series with its respec tive resistor characterized in that the system can detect a voltage proportional to the armature current through the transistors (30, 31, 32) to provide a signal to be used to render said transistors non-conducting when said proportional voltage exceeds a predetermined value, said system comprising diodes (36, 37, 38) having like terminals connected to a'common point (28) at which the proportional voltage to the armature current can be detected and having their other terminals branched each connected to one of said transistors (30, 31, 32) between the transistor and its series resistor (33, 34, 35).

7. Electric traction system comprising a separate excitation d.c. motor, an armature current control device, an excitation current control device, and between the motor armature and the d.c. supply source, several armature current switches (4-1...4-n) in parallel each made up of one or more transistors connected in parallel (30, 31, 32) characterized in that the armature current control device comprises an electronic flip-flop (12) with two stable electric states controlling the transistor or transistors (39) for controlling the armature current switches (4-1...4-n) made up of transistors in parallel (30, 31, 32) and three pulse generatorsoperating said electronic flip-flop (12), namely, a fixed frequency pulse generator (10) cyclically imposing the closing of the switches (4-1...4-n), a variable frequency pulse generator (9) controlling the opening, at will, of these switches, a threshold pulse generator (11) causing the automatic opening of said switches protecting them against any predetermined overload.

8. Electric traction system according to claim 7 wherein the variable frequency pulse generator (9) is provided with a unijunction transistor (44) and a capacitance (52) shunting the emitter (50) and the base (45) of this transistor and whose discharge through the emitter (50) base (45) region of this transistor gives a pulse at its output (21), the base (45) being connected to the negative pole of the current source (3), characterized in that the variable frequency pulse generator (9) comprises between the capacitance (52) emitter I (50) junction point of the unijunction transistor (44) and the positive pole of the current source (3), a potentiometer (19) variable by means of a pedal (20).

9. Electric traction system according to claim 7 wherein the variable frequency pulse generator (9) is provided with a unijunction transistor (44) and a capacitance (52) shunting the emitter (50) and the base (45) of this transistor and whose discharge through the emitter (50) base (45) region gives a pulse at its output (21), the base (45) being connected to the negative pole of the current source (3), characterized in that this variable frequency pulse generator (9) comprises a transistor (53) whose collector and emitter shunting the capacitance (52) and the base is connected through a diode (54) to the terminal (23) of this generator (9) which is connected to the output (17) of the fixed frequency pulse generator (10).

10. Electric traction system according to claim 7 wherein the threshold pulse generator (11) is provided with a unijunction transistor (55) and a capacitance (61) shunting the emitter and the base (56) ofthis transistor and whose charge is controlled by the potential of a terminal (26) connected to the output (15) of the electronic flip-flop (l2) and whose discharge through the emitter (60) base (56) region gives a pulse at its output (24), the base (56) being connected to the negative pole of the current source (3), characterized in that this threshold pulse generator comprises, on the one hand, a potentiometer (27) connected through a diode to the junction point of the capacitance (61) and the emitter (60) of the unijunction transistor, and, on the other hand, a transistor (66) whose collector and emitter shunting the bases (56, 57) of the unijunction transistor (55) and whose base is connected to a terminal (25) connected to the common point (28) of the cathodes of the diodes (36, 37,

38) mounted below the armature current switches. 

1. Electric traction system comprising a separate excitation d.c. motor, an armature current control device, an excitation current control device, and between the motor armature and the d.c. supply source, several armature current switches (4-1...4-n) in parallel each made up of one or more transistors connected in parallel (30, 31, 32) characterized in that in order to avoid possible non-synchronization of the switches each switch is connected at one terminal to an induction coil (5-1...5-n) and in series with the armature of said motor, and at the other terminal with a diode (8-1...8-n) shunting said induction coil and armature of the motor.
 2. Electric traction system according to claim 1 wherein each of the armature current switches in series to its respective resistor (7-1...7-4) characterized in that in order to provide a signal to open the switches when a voltage proportional to the armature current passing through the switches is too large and exceeds a predetermined value, the system includes diodes (21-1...21-n) having like terminals connected to a common point (28) where the voltage proportional to the armature current may be detected, and whose other terminals are branched each being connected to one of the armature current switches between the switch and its series resistance (7-1...or 7-n).
 3. Electric traction system according to claim 2 characterized in that the excitation current control device comprises, on the one hand, at least a transistor (70) acting as the excitation current switch and a threshold system (72) which, connected its output (73) to the base of the transistor (70) controls the conduction of said transistor, in other words, the opening and closing of this excitation current switch, and, on tHe other hand, a unit comprising at least a saw-toothed pulse generator (76), an amplifier (77) of the voltage proportional to the armature current, taken from the common point (28) of the cathodes of the diodes (36, 37, 38) mounted below the armature current switches and an adder (78) whose two inputs (80 and 83) are respectively connected to the output (79) of said saw-toothed generator and to the junction point between the charge capacitance (82) and the output of said amplifier (77) and whose output (84) is connected to the input (74) of this threshold system (72).
 4. Electric traction system according to claim 3, characterized in that it comprises in the control of the threshold system (72), on the one hand, a potentiometer (86) giving the threshold voltage and whose slide contact is connected through a diode (85) to the input (75) of a threshold system (72) and, on the other hand, a unit comprising a potentiometer (87) whose slide contact is controlled by an acceleration pedal (20) and connected through a diode (88) to the input (75) of the threshold system (72), a threshold system (89) feeding by its output (92) said potentiometer (87) as soon as a tachymetric element (91), connected to its input (10), displays a predetermined rotation condition of the armature (1) of the motor.
 5. Electric traction system according to claim 3 provided with an energy recovery system during braking, characterized in that this energy recovery system comprises, on the one hand, a recovery diode (92) whose anode is connected to the negative pole of the current source (3) and the cathode is connected to the common point of the induction coils (5-1...5-n) and the terminal of the armature (1) of the motor and, on the other hand, a subassembly made up of a voltage comparator (93), a potentiometer (94) whose slide contact is connected to a brake pedal (95) and an input of this comparator (93), a connecting diode (97) whose anode is connected to another input of the comparator (93) and the cathode to the cathode of the recovery diode (92) and an amplifier (96) whose input is connected to the output of the comparator (93) and the output is connected to the input (83) of the adder (78) of the excitation current control device of the motor of the electric traction system.
 6. Electric traction system according to claim 1 wherein (each of) the armature current switches has transistors (30, 31, 32) connected in parallel and each of said transistors is connected in series with its respective resistor characterized in that the system can detect a voltage proportional to the armature current through the transistors (30, 31, 32) to provide a signal to be used to render said transistors non-conducting when said proportional voltage exceeds a predetermined value, said system comprising diodes (36, 37, 38) having like terminals connected to a common point (28) at which the proportional voltage to the armature current can be detected and having their other terminals branched each connected to one of said transistors (30, 31, 32) between the transistor and its series resistor (33, 34, 35).
 7. Electric traction system comprising a separate excitation d.c. motor, an armature current control device, an excitation current control device, and between the motor armature and the d.c. supply source, several armature current switches (4-1...4-n) in parallel each made up of one or more transistors connected in parallel (30, 31, 32) characterized in that the armature current control device comprises an electronic flip-flop (12) with two stable electric states controlling the transistor or transistors (39) for controlling the armature current switches (4-1...4-n) made up of transistors in parallel (30, 31, 32) and three pulse generators operating said electronic flip-flop (12), namely, a fixed frequency pulse generator (10) cyclically imposing the closing of the switches (4-1...4-n), a variable frequency pulse generator (9) controlling the opening, at will, of these switches, a Threshold pulse generator (11) causing the automatic opening of said switches protecting them against any predetermined overload.
 8. Electric traction system according to claim 7 wherein the variable frequency pulse generator (9) is provided with a unijunction transistor (44) and a capacitance (52) shunting the emitter (50) and the base (45) of this transistor and whose discharge through the emitter (50) base (45) region of this transistor gives a pulse at its output (21), the base (45) being connected to the negative pole of the current source (3), characterized in that the variable frequency pulse generator (9) comprises between the capacitance (52) - emitter (50) junction point of the unijunction transistor (44) and the positive pole of the current source (3), a potentiometer (19) variable by means of a pedal (20).
 9. Electric traction system according to claim 7 wherein the variable frequency pulse generator (9) is provided with a unijunction transistor (44) and a capacitance (52) shunting the emitter (50) and the base (45) of this transistor and whose discharge through the emitter (50) - base (45) region gives a pulse at its output (21), the base (45) being connected to the negative pole of the current source (3), characterized in that this variable frequency pulse generator (9) comprises a transistor (53) whose collector and emitter shunting the capacitance (52) and the base is connected through a diode (54) to the terminal (23) of this generator (9) which is connected to the output (17) of the fixed frequency pulse generator (10).
 10. Electric traction system according to claim 7 wherein the threshold pulse generator (11) is provided with a unijunction transistor (55) and a capacitance (61) shunting the emitter (60) and the base (56) of this transistor and whose charge is controlled by the potential of a terminal (26) connected to the output (15) of the electronic flip-flop (12) and whose discharge through the emitter (60) - base (56) region gives a pulse at its output (24), the base (56) being connected to the negative pole of the current source (3), characterized in that this threshold pulse generator comprises, on the one hand, a potentiometer (27) connected through a diode (65) to the junction point of the capacitance (61) and the emitter (60) of the unijunction transistor, and, on the other hand, a transistor (66) whose collector and emitter shunting the bases (56, 57) of the unijunction transistor (55) and whose base is connected to a terminal (25) connected to the common point (28) of the cathodes of the diodes (36, 37, 38) mounted below the armature current switches. 